Brown, D-Ohio, and Sen. Chuck Grassley, R-Iowa, sent a letter to U.S. Attorney General Eric Holder questioning the federal government can effectively prosecute banks that have been deemed "too big to fail."

Specifically, they want to know if the federal government can prosecute those banks and impose penalties on them. They also want Justice Department to disclose the identities of parties that prosecutors consult with about the appropriate level of penalties for financial institutions.

"Already, the nation's six largest megabanks enjoy what amounts to taxpayer-funded guarantee by virtue of their size, making it harder for regional and community banks to compete," Brown, a member of the Senate Banking Committee, said in a statement. "Now, these megabanks may also enjoy some impunity when they violate the law by laundering money or illegally foreclosing on homeowners. Wall Street should pay the full price of its wrongdoing, not pass the costs along to taxpayers."

"The large number of private and government lawsuits since the global financial crisis continues to undermine public confidence in our financial markets," the two senators wrote to Attorney General Eric Holder. "This confidence can only be restored by demonstrating that there are consistent rules in place that provide accountability for wrongdoing and deter financial predators…. Our markets will only function efficiently if participants believe that all laws will be enforced consistently, and that violators will be punished to the fullest extent of the law. There should not be one set of rules that apply to Wall Street and another set for the rest of us.